Baghdad — A roadside bomb hit the convoy of the top UN official in Iraq after a meeting with revered Shiite cleric Grand Ayatollah Ali al-Sistani in Najaf Tuesday. UN envoy Ad Melkert was unhurt, but the blast killed an Iraqi policeman and wounded three others.

A UN spokeswoman said the bomb went off as Mr. Melkert’s convoy, accompanied by an Iraqi security detail, headed to the Najaf airport Tuesday afternoon on the outskirts of the city.

Mr. Melkert, the special representative to the UN secretary general, is one of the few Western officials with whom Ayatollah Sistani meets. The cleric, who does not appear in public, has played an influential role in Iraq. Although he avoids direct involvement in politics, his edicts helped lead to direct elections in 2005 and strong voter turnout, including by women. He has also directed followers not to retaliate following sectarian attacks that have rocked the country.

Melkert was shown on Iraqi television emerging from the meeting at Sistani’s modest home urging political leaders to quickly come to an agreement on forming a new government. Seven months after Iraqis went to the polls, a deadlock among political leaders has prompted fears that violence could increase if a government which includes Shiites, Sunnis, and Kurds isn’t formed soon.

Najaf, home to one of the holiest sites in Shiite Islam, as well as one of its four main centers for religious learning, is traditionally much calmer than Baghdad. Access to the city is tightly controlled through a series of checkpoints to protect hundreds of thousands of religious pilgrims, many of them Iranian.

Amid what appears to be an increase in violence in the normally calm south, Iranian authorities recently called on the Iraqi government to increase security for Iranian pilgrims following several attacks against travelers.

The United Nations has operated under extremely tight security in Baghdad’s green zone since a truck bomb at its headquarters in 2003 killed 22 UN staff, including top envoy Sergio Vieira de Mello.